This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-274880, filed Sep. 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an image formation apparatus which forms an image using multiple laser beams and more specifically to a multi-beam control method for controlling the positions in the sub-scanning direction of the laser beams on a photosensitive drum with galvanomirrors and an image formation apparatus which uses the control method.
A multi-beam optical system is equipped with a plurality of laser beam sources and a single polygon mirror and is required to perform beam position control in order to prevent image misalignment due to the relative displacement of beams in the image plane. In this specification, xe2x80x9cbeam positionxe2x80x9d refers to the position of each beam in the sub-scanning direction that is scanned across the image plane, such as the surface of the photosensitive drum, in the main scanning direction.
Methods of controlling the positions of beams in the sub-scanning direction include a method of setting the spacing between each beam through the use of a plurality of galvanomirrors each having its reflecting surface (mirror) arranged to be rotatable in any direction as disclosed in, for example, Japanese Unexamined Patent Publication No. 10-76704 and Japanese Unexamined Patent Application No. 2000-147398.
In the sub-scanning control in the multi-beam optical system, coarse adjustment is made first. The coarse adjustment is intended to, when a beam is greatly off a target course and hence does not pass over a photosensor, in which case beam fine adjustment can not be made, shift the beam over the photosensor.
In general, in the coarse adjustment, a directive value, for example, a lower limiting directive value, is set in the galvanomirror and then the setting value is changed in large steps toward an upper limiting directive value to shift the transit position of a beam in the sub-scanning direction. For example, a step size of 100 bits results in a change of 176 xcexcm in the image plane position. As shown in FIGS. 17A and 17B, a beam reflected by the galvanomirror is directed onto the surface of the photosensitive drum to form a latent image on it. The latent image is developed and then transferred onto paper. The sensor surface and the paper surface are set equidistant from the galvanomirror. The amount by which the beam position is displaced on the sensor surface appears on paper as it is. Thus, the beam position on the sensor surface is also called the image plane position.
FIG. 18 is a flowchart for the conventional coarse adjustment. This coarse adjustment is made automatically by the apparatus. First, of four beams, a beam to be adjusted is turned on (step S110). Next, a determination is made as to whether horizontal synchronization (HSYNC) is established (step S101). If established, then the coarse adjustment of the next beam is made.
If synchronization is not detected in step S101, then a lower limiting directive value is first set in the galvanomirror so that it is turned below the image plane at the maximum swing angle and then the setting value is increased in steps of 100 bits (corresponding to about 176 xcexcm) (step S106).
The above operation is repeated until HSYNC has been established for all the beams. The control is repeated in the order of, for example, beam 1, beam 2, beam 3, beam 4, beam 1 and so on. If there are beams for which adjustment has been made, they are skipped and the control is repeated in the order of, for example, beam 1, beam 3, beam 4, beam 1 and so on. When, even if the setting value is changed until the upper limiting value is reached, no HSYNC is detected, repair is needed. In this case, the necessity for service call is displayed on the apparatus.
After the termination of the coarse adjustment for all the beams, fine adjustment is made. FIG. 19 is a flowchart for the conventional fine adjustment. The fine adjustment is made to drive the beam which has been allowed to pass over the sensor surface by the coarse adjustment into a target range of xc2x110 xcexcm of a target value. The fine adjustment is essential in obtaining correct output images.
As with the coarse adjustment, only a beam to be controlled is turned on (step S110). The beam position information in the sensor is read (step S111). A determination is made as to whether the position over which the beam passes is within the target range (step S112). If the beam position lies within the target range, then the adjustment is terminated; otherwise, the galvanomirror is instructed so as to change the beam position on the sensor surface in small steps, thereby driving the beam position into the target range (step S113).
FIG. 20 shows a plot of output versus input of the galvanomirror. The input to the galvanomirror is given in voltage. Voltages from xe2x88x9210.6 to +10.6V are made correspond to 0EB8H to 3333H in hexadecimal notation. These hexadecimal numbers are handled as input values to the galvanomirror. These input values are converted into voltages by a D/A converter and then applied to the galvanomirror. The input values below 0EB8H or above 3333H correspond to voltages in the vicinity of supply voltages (xc2x112V). For these input values, the output of the A/D converter is not proportional to the input. Thus, these input values are not generally used.
That is, 1 bit corresponds to an input voltage of 16 mV. The output is given in swing angles of the galvanomirror from xe2x88x9239.46 mrad to +39.46 mrad. In correspondence to these swing angles, the image plane position changes from xe2x88x928.68 mm to +8.68 mm. Namely, when the input value is changed by 1 bit, the beam position on the image plane is changed by 1.76 xcexcm.
For countermeasures against the galvanomirror being heated, the use of 5% portions of the driving range at both ends thereof is prohibited by software. Thus, the actual input ranges from xe2x88x9210.1 to +10.1 V and the output ranges from xe2x88x9237.49 to +37.49 mrad (the amount by which the image plane position is changed from xe2x88x928.25 to +8.25 mm).
FIG. 21 illustrates the direction of operation of the galvanomirror, variations in the beam direction, and input data (0EB8H to 3146H) and input voltages (xe2x88x9210.1 to +10.1V) to the galvanomirror. When the input value is changed from 0EB8H to 3146H, the input voltage changes from xe2x88x9210.1V to +10.1V and the direction of the beam changes upward by 74.98 mrad.
The galvanomirrors have characteristics that greatly vary from galvanomirror to galvanomirror. As shown in FIG. 22, the amount by which the image plane position is changed per unit change (1 bit) in the input varies from 1.23 to 2.22 xcexcm, the maximum distance moved by the image plane varies from 12.11 to 21.86 mm, the swing angle per unit change in the input varies from 6.22 to 11.23 xcexcrad, and the maximum swing angle varies from 55.04 to 99.38 mrad. Thus, the galvanomirrors contain an individual difference of about xc2x130% in their characteristics.
The time of response to input of a directive value greatly varies from galvanomirror to galvanomirror if only the mixing ratio of dumping materials varies slightly. FIGS. 23, 24 and 25 show response characteristics when the mixing ratio is 1:1.1, 1:1.2, and 1:1.3, respectively. The time (mS) is shown on the horizontal axis and the amount (xcexcm) by which the image plane position is changed is shown on the vertical axis. A change in the image plane position about 10 mS, the data sampling interval in conventional control, after the galvanomirror has been given a directive value is about 130 xcexcm in FIG. 23, about 100 xcexcm in FIG. 24, and about 80 xcexcm in FIG. 25. It will therefore be seen that a little change in the mixing ratio results in variations in the response characteristic of the galvanomirror.
In Japanese Unexamined Patent Application No. 2000-147398, the galvanomirror used in sub-scanning control is composed of a magnet fixing base, a magnet, a bobbin, a yoke, a coil, a torsion bar, and a mirror. The mirror and the torsion bar are glued together at four points. For this reason, a great movement of the mirror may result in the glue peeling off and the reduced life of the galvanomirror.
The problems with the conventional control are listed below.
In the conventional control, since the maximum voltage (xe2x88x9210.1V) is applied to the galvanomirror, the load on the galvanomirror increases.
The coarse adjustment is made in order to search for the sensor which should be able to be found with close to the directive value (close to 0V) used in the last control. However, the lower limiting value (xe2x88x9210.1V) is set in the galvanomirror and the beam is then moved in units of 100 bits until it comes to pass over the sensor. That is, the conventional control method is performed in such a way as to search for a nearby object from the distance, which involves waste of time.
The conventional control is not performed so as to accommodate the individual difference among galvanomirrors in the change in the image plane position per unit change (1 bit) in the directive value. That is, the same control is performed on a galvanomirror for which the change in the image plane position per unit change in the input is a minimum of 1.23 xcexcm and a galvanomirror for which the change in the image plane position per unit change in the input is a maximum of 2.22 xcexcm. For example, when an attempt is made to move the image plane position by 176 xcexcm (corresponding to 100 bits for the average product), the change in the image plane position is as small as 123 xcexcm for the minimum product and as large as 222 xcexcm for the maximum product. Thus, further movement is needed in either case, involving waste of time. In addition, with the maximum product, there is the possibility that the image plane may jump over its target position and become uncontrollable.
The conventional control is not performed so as to accommodate the individual difference among galvanomirrors in the response time since a directive value was applied. That is, the same control is performed on a galvanomirror for which the degree of achievement about 10 mS after a directive value has been given is 80%, a galvanomirror which is 70% in the degree of achievement, and a galvanomirror which is 60% in the degree of achievement. For example, if control is performed on the assumption that the degree of achievement is 70% and the actual degree of achievement is 80% or 60%, then overshoot or shortage of movement will occur, which requires further movement and involves waste of time.
As described above, with the conventional control, since the characteristics of individual galvanomirrors, such as the change in the image plane position per unit change in an input directive value and the response time, are unknown, the time of control cannot be estimated and a large time margin has to be ensured. In general, after having been warmed up, the drum continues rotating during control of the optical system, resulting in increased idle running time of it.
It is therefore a first object of the present invention to prevent a heavy load from being imposed on galvanomirrors during laser beam sub-scanning direction position control.
It is a second object of the present invention to eliminate useless control during laser beam sub-scanning direction position control.
It is a third object of the present invention to perform a control operation to conform to differences among galvanomirrors in change in the image plane position to thereby reduce the required control time.
It is a fourth object of the present invention to perform a control operation to conform to differences among galvanomirrors in response time to thereby reduce the required control time.
It is a fifth object of the present invention to grasp the differences among galvanomirrors, reduce margin in galvanomirror control, and reduce the required time of control.
In order to achieve the above objects, according to one aspect the present invention, there is provided a method for controlling the sub-scanning direction beam position, for use with an image forming apparatus including multiple laser sources, a single polygon mirror, galvanomirrors, and a laser beam position detecting optical sensor, comprising the steps of: controlling the galvanomirror with the last galvanomirror directive value when sub-scanning beam position control was performed in the last operation of the image forming apparatus; a first decision step of deciding whether the sub-scanning direction beam position has reached a target range by referring to an output value of the sensor; estimating the drift direction of the galvanomirrors on a basis of the last directive value, when the sub-scanning direction beam position has not reached the target range with the last directive value; providing directive values to the galvanomirror so that the sub-scanning direction beam position is moved in steps from the position controlled by the last directive value in the estimated direction; determining whether the directive value provided to the galvanomirror has reached a predetermined value; and providing directive values to the galvanomirror so that the sub-scanning direction beam position is moved in steps from the position controlled by the last directive value in the direction opposite to the estimated direction, when the directive value have reached the predetermined value.
Thus, when the beam position does not fall within a target range, the beam is shifted always in either plus or minus direction from the directive value in the last operation, preventing a heavy load from being imposed on the galvanomirrors. A drift in the last operation is estimated and the beam is shifted from the direction to correct the drift, thus eliminating useless control during the beam sub-scanning direction position control.
According to another aspect the present invention, there is provided a method for controlling the sub-scanning direction beam position, for use with an image forming apparatus including multiple laser sources, a single polygon mirror, galvanomirrors, and a laser beam position detecting optical sensors, comprising the steps of: measuring an movement amount of a beam after predetermined directive value has been provided to the galvanomirror at regular time intervals using changes in outputs of the sensors; determining a final movement amount of the beam and response times of the galvanomirror for the directive value from the measurements of the movement amount of the beam; and controlling the beam position in the sub-scanning direction using the final movement amount of the beam and the response times.
Further, the step of controlling the beam position includes a step of making fine adjustment of the beam position, and the fine adjustment step further includes a step of turning on a beam to be controlled and scanning the beam in the main scanning direction, a step of deciding the sub-scanning direction position of the beam by reading the outputs of the optical sensors, a step of, when the sub-scanning direction position is not within a target range, calculating a specific directive value on the basis of the final movement amount of the beam for the directive value and providing the specific directive value to the corresponding galvanomirror, and a step of deciding the sub-scanning direction position of the beam to be controlled after the response time for fine adjustment since the specific directive value was provided to the galvanomirror.
Thus, the movement amount and the response time for the directive value are determined for each galvanomirror and, using the movement amount and the response time, the fine adjustment of the beam position is made. Therefore, the control time can be reduced, the total control time can be estimated, and the overall efficiency of the beam control can be increased.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.